Chuck magnetizing and demagnetizing control system



8 9 l VQ' 9 2 5 .,M 2E T S v. s L o R T N 0 C m T. Z I m w m D m G N I Z I m G A M K m C H. F. STORM Nov. 7, 1950 2 Sheets-Sheet 1 Filed Feb. 10, 1949 NMIS Inventar; Her-'bert F`, Storm,

wf'ym r-i ls Attorney.

H. F. STORM Nov. 7, 1950 CHUCK MAGNETIZING AND DEMAGNETIZING CONTROL SYSTEM Filed Feb. l0, 1949 2 Sheets-Sheet 2 1r" l THYRATRon Inventor: Herbert F`. Storm,

T..Euzmau V626 L nf/f by Hls Attorney.

Patented Nov. 7, 1950 CHUCK MAGNETIZING AND DEMAG- NETIZING CONTROL SYSTEM Herbert F. Storm, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application February 10, 1949, Serial No. 75,557

Claims.

Ihis invention relates to control systems, more particularly to control systems for magnetizing and automatically demagnetizing electromagnetic chucks and it has for an object the provision of a simple, reliable and improved control system of this character.

Electromagnetic chucks require D.C. for energization. If the workpiece is to be removed from the chuck, the D.C. energization must be interrupted and the residual magnetism must be removed. To remove the residual magnetism the magnetic chuck may be energized by D,C. in the reverse direction, for example, by means of a double pole, double throw switch. The duration of the reversed current ow must be very accurately controlled. If the duration is too short, some of the residual will remain, and if the duration is too long, the chuck will magnetize with reverse polarity which will produce residual magnetism also of reverse polarity.

It is not unusual for an operator of a magnetic chuck to make a number of reversals with the reversing switch and finally remove the workpiece from the chuck with the aid of a sledge hammer.

Both the arcing at the tips of the double pole, double throw switch and the haphazard procedure of demagnetization are undesirable from the point oi View of eiiicient shop practice. Furthermore, a source of D.C. supply is often unavailable. Consequently further objects of the invention are the provision of a chuck magnetizing and demagnetizing control system which is energized from an A.C. source and in Which the demagnetization is automatic and complete to any desired degree and arcing at the tips of the double pole, double throw switch is eliminated.

In carrying the invention into eiTect in one form thereof, the magnetizing coil of a magnetic chuck is supplied from a half wave rectifier with which it is connected in series to a source of single phase alternating voltage. The rectifier is preferably a thyratron which is provided with an anode, a cathode and a control grid. Across the chuck coil is a reversely connected diode valve which maintains the current ilow in the chuck coil in that part of the'cycle during which the thyratron does not conduct. Across a source of direct voltage which is preferably supplied through a rectifier from. the alternating voltage source is connected a capacitor and a potentiometer in series with each other. A control switch has one operating. position in which it renders the capacitor inactive and connects the grid of the thyratron to the slider of the potentiometer to eiTect conduction and another position in which it renders the capacitor active and connects the grid of the thyratron to a source of negative bias voltage to terminate conduction by the thyratron and to initiate decay of the current in the coil through the diode valve. A relay7 responsive to the disappearance of the arc drop voltage across the diode reverses the connection of the chuck coil to the output circuit of the thyratron and reestablishes the connection of the grid ol the thyratron to the potentiometer.

For a better and more complete understanding of the invention reference should now be had to the following specification and to the accompanying drawings, of which Fig. l isfa simple diagrammatic sketch of an embodiment of the invention and Figs. 2 and 3 are charts of characteristic curves which serve to facilitate an understanding of the invention.

Referring now to the drawing, the magnetizing coil I of a magnetic chuck is to be energized from a source of alternating voltage which is represented by the supply conductors 2 and 3. The chuck is also to be demagnetized by current supplied from the alternating voltage source. An electric valve li, which is preferably a thyratron, is connected between the chuck coil and the supply conductor 2. The anode 4a of the thyratron is connected to the supply conductor 2 and the cathode Lib is connected to the upper input terminal 5a of a ratcheting type reversing switch 5 of which the lower input terminal 5b is connected to the other supply conductor 3. A diode electric valve l5 has its anode 6a connected to the switch terminal 5b and its cathode 6b connected to the terminal 5a. Thus with the switch 5 closed for either direction of magnetization, the thyratron is connected in series with the chuck magnetizing coil and the diode 6 is reversely connected in parallel with the coil.

A combined alternating voltage and,l direct voltage is supplied between the grid and cathode of the thyratron. The direct voltage component is obtained from the supply busses l and 8 which in turn obtain -the direct voltage from a voltage doubler circuit comprising the diode valves 9 and I0 and the capacitors H and l2. The voltage doubling circuit is energized from the secondary winding I3b of a transformer I3 of which the primary winding i3d is connected across the A.C. supply conductors 2 and 3.

Across the direct voltage busses 'I and 8 a capacitor I4 and a potentiometer I5 are connected in series. One pair of contacts ia and Ib of a double pole, double throw switch is connected in parallel with the capacitor It and short-circuits the capacitor when the switch is in the up or magnetize position. Also in the up position of the switch, the movable contact IEc engages the upper stationary contact itd to connect the grid dc of the thyratron through the secondary winding I'Ib of the grid transformer H to the slider Ia of the potentiometer i5. In the down or de-magnetize position of the switch theqnovable contact Ic connects the grid through the normally closed contacts 18d or" an auxiliary relay to the conductor 8 which is negative with respect to the conductor I9 by a substantial amount, e. g. 145 volts.

The primary winding Ilia of the grid transformer is connected across the capacitor 2Q of 'a phase shifting network which comprises the capacitor 2D and a resistor 20a connected across the lsecondary winding i3b of the transformer I3. The voltage induced in the secondary winding Hb of the grid vtransformer lags the anode voltage of the thyratron approximately 70. By increasing the direct voltage bias in a positive direction, the firing point of the thyratron .is advanced and more current ilows through the magnetizing coil of the chuck.

Across a suitable source .of alternating voltage, such as represented by the secondary winding 2 I a of a transformer .2-I, the resistor 22, the operating coil 5c of the ratchet Atype reversing switch, and a control electric valve are connected in series. Across the resistor 22 the operating coil of the auxiliary relay is connected `through its own normally closed contacts Ic. A resistor 2li is connected in series with `the relay coil and a capacitor 25 is connected in parallel with the relay coil.

With the foregoing understanding of the ele- `ments and their organization, the `operation of the system will readily be understood. The operation of the thyratron and diode G will Yirst be considered. The alternating voltage .of the source across the `conductors 2 and 3 is represented in Fig. 2A by the sinusoidal curve 26 `and the voltage across the thyratron is represented by the curve 2l'. Assuming the iring .angle to be as shown, the thyratron will begin to conduct at vpoint a. In Fig. 2B the voltage .of the source is again represented by lcurve 25 andthe voltage across the diode valve ii is represented by the curve 28 considering the voltage of the :anode Ea as the base line and the curve 28 representing the yvoltage of the cathode. Between points b and c the voltage of the cathode ib becomes negative with respect to 'the anode voltage `and conduction of valve 6 becomes feasible assoon as the voltage attains the firing Volta-ge Ef. W hen this occurs at point c, the diode begins to conduct and the voltage across the diode assumes the value-oi the arc drop voltage EEE.. At fthe same time the plate cathode voltage .2is of 'the thyratron becomes less than the arc drop voltage EaI and conduction in the thyratron is extinguished. The chuck current now flows through the diode, as shown in bythe curve i diode '6 (where 'the current ripple is vneglected) until the thyratron res againatd. The

iinverse Voltage across the diode again becomes positive and extinguishes conduction in the diode.

In order to demagnetize the chuck, the direction of the D.C. magnetization current is red Y versed and the intensity of the current is at the same time reduced. This action is repeated a number of times until the chuck is demagnetized. The reversal of current and its decay are automatically performed by the control system of Fig. l in the following manner.

rlhe switch IG is moved to the down or demagnetize' position. `The movable contact iiic connects the grid c of the thyratron d to the negative bus 8 through the normally closed con tacts ISa of the auxiliary relay. Thereafter the thyratron cannot reignite and the chuck current which flows through the diode 6 begins to decrease, as illustrated by the curve 3i in Fig. 3. rihe decrease which is indicated by the portion oi the curve between points e and f continues until the magneticenergy of the chuck has been dissipated in the circuit. When the magnetic energy 'has been dissipated, the diode ceases conducting and the arc drop voltage EaZ across the diode vanishes. This disappearance of arc drop voltage y'1s used as a signal yto reverse the polarity of the current flow lin the chuck coil. The primary winding Zio of transformer '2i is connected across the supply conductors Z and 3 with such polarity that the voltage of the conductor 32 is positive with respect to conductor 3 when an arc drop voltage Ea? appears across the diode valve, as indicated in Fig. 2, B and F.

As can be seen from Fig. l, the voltage across the diode valve is supplied between the grid and cathode of the control valve 23. As long as there is an arc drop voltage Ea? across the diode valve, the grid cathode voltage of valve 23 is negative and no conduction takes place. When the chuck current ceases to flow at the point f in Fig. 3, the arc drop voltage EQ2 disappears and consequently the control valve 23 loses its negative bias and begins to conduct. in response to conduction by the control valve, the operating coil 5c of the reversing switch is energized and its double pole, double throw contacts are actuated to the reverse position to pre pare for a reverse energization oi the magnetiZing coil of the chuck.

When valve 23 begins to conduct at point f, Fig. 3, a voltage appears across the resistor 22. The build-up of the Voltage across the coil of the auxiliary relay IS is delayed by the parallel capacitor 25, so that a delay is obtained between the operation of the ratchet reversing -contactor 5 and the response of the auxiliary relay. 1n responding, the auxiliary relay opens its contacts Ia to remove the negative bias from the v grid of the thyratron and closes its contacts Itib to connect the grid to the slider Ia Vof the potentiometer. The grid of the thyratron now receives a voltage which enables it to conduct.

When the switch VI6 was operated `to initiate the demagnetization, the capacitor I4 began to charge in response to removal of the kshort circuit by the opening of contacts Ita ,and Ib. Consequently vthe direct lvoltage at the slider l5ais decreased from the value that existed at the instant of opening the switch I6. The current in the chuck increases from Ag toward h.

`The delay effected by the charging of capacitor 25 is Arepresented by the discontinuity in lthe current of the chuck coil between the points f and g. As noted in Fig. 3, the magnitude of the chuck current is less at point h than at point e owing to lthe reduced direct voltage on the grid of the thyratron resulting from the charging of capacitor I4. If the auxiliary Vrelay vwould remain picked up, the ychuck current would decay along the dotted curve 33 as the capacitor I 4 continued to charge. However, in responding to energization, the auxiliary relay opened its normally closed contact I8c. The auxiliary relay remains picked up for an interval of time determined by the discharge current of capacitor 25 and then drops out at a time corresponding to the point h in Fig. 3. In dropping out, the auxiliary relay opens its contacts I8b and closes its contacts Ia to reconnect the grid of the thyratron to the source of negative bias. As a result the thyratron becomes non-conducting and the chuck coil current flows through the diode and decays in accordance with the portion *of the curve between h and f in Fig. 3. This operation is repeated between the points g and f and between the points g" and f", etc., as indicated in Fig. 3. With each reversal the magnitude of the maximum value of the chuck current decreases from the previous maximum owing to the charging of the capacitor I4 andthe effect of its increasing charge on the conduction of the thyratron. As shown in v the example of Fig. 3, the chuck is completely demagnetized after iive reversals at the point f" and the workpiece may be easily removed.

'Ihe time required for the chuck current to decay during each reversal is not fixed butdepends upon the magnetic property and shape of the workpiece. Consequently no time is wasted between reversals since each reversal begins at the end of the previous decay and not at the end of a fixed time which must be equal to the longest decay that will be encountered.

Another advantage of the system is obtained when the chuck is operated at less than maximum induction. For example, assume that the chuck is energized with a steady state current of a magnitude equal to h' in Fig. 3. When the switch I6 is moved to the demagnetize position7 the current will decay from h to j and only two more reversals will follow before the chuck is completely demagnetized.

Although in accordance ,with the provisions of the patent statutes this invention is described as embodied in concrete form and the principle thereof has been explained together with the best mode in which it is now contemplated applying that principle, it will be understood that the elements shown and described are merely illustrative and that the invention is not limited thereto since alterations and modications will readily suggest themselves to persons skilled in the art without departing from the true spirit of this invention or from the scope of the annexed claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A chuck magnetizing and demagnetizing control system comprising a pair of alternating voltage supply conductors, a pair of chuck magnetizing coil terminals, a first electric valve provided with an anode, a cathode and a control grid and having its anode connected to one of said supply conductors and its cathode connected to one of said terminals, a connection from the other of said terminals to the other of said conductors, an electromagnetic reversing switch having an operating coil and having reversing contacts controlled thereby and connected to said terminals for reversing the connections of said terminals to a chuck magnetizing coil, a diode electric valve having its anode connected to said other conductor and having its cathode connected to the cathode of said rst electric valve, a source of direct voltage, a capacitor and a potentiometer connected in series across said source, switching means operable to one position to short-circuit said capacitor and to establish a connection from said grid to said'potentiometer and operable to a second position to interrupt said short circuit and to connect said grid to a source of negative -bias to render said rst'valve nonconducting and to initiate decay of. the current of said `chuck coil through said diode valve, an electric valve having an anode-cathode circuit connected to said operating coil and a grid cathode circuit connected across said diode valve to be responsive to disappearance of the arc drop voltage across said second valve for energizing said reversing switch to reverse said chuck coil connections and for reestablishing said connection from said grid to said potentiometer.

2. A chuck magnetizing and demagnetizing control system comprising a pair of alternating voltage supply conductors, first and second chuck magnetizing coil terminals, a rst electric valve provided with an anode, a cathode and a control grid and having its anode connected to one of said supply conductors and its cathode connect- `ed to said first terminal, a connection from said second terminal to said other supply conductor, a diode electric valve having its cathode connected to said i'lrst terminal and its anode connected to said other supply conductor, a source of direct voltage, a capacitor and a potentiometer Connected in series across said source, switching means operable to one position to render said capacitor inactive and to establish a connection from said grid to said potentiometer to initiate conduction in said rst valve to magnetize said coil and operable to a second position to render said capacitor active and to connect said grid to a source of negative bias voltage to interrupt conduction of said rst valve and to initiate decay of the current in said chuck coil through said diode valve, and means cornprising a switching device controlled in response to-decay of the current of said magnetizing coil for reversing the connections to said coil and for removing said negative bias and reestablishing said connection from said grid to said potentiometer. i

3. A chuck magnetizing and demagnetizing control system comprising a pair of alternating voltage supply conductors, rst and second chuck magnetizing coil terminals, a iirst electric valve provided with an anode, a cathode and a control grid and having its anode connected to one of said supply conductors and its cathode connected to said rst terminal, a connection from said second terminal to said other supply conductor, a diode electric valve having its cathode connected to said rst terminal and its anode connected to said other supply conductor, a source of direct voltage, a capacitor and a potentiometer connected in series across said source, switching means operable to one position to render said capacitor inactive and to establish a connection from said grid to said potentiometer to initiate conduction in said first valve to magnetize said coil and operable to a second position to render said capacitor active and to connect said grid to a source of negative bias voltage to interrupt conduction of said rst valve and t0 initiate decay of the current in said chuck coil through said diode valve, a control electric valve having an anode, a cathode and a v:control grid, Aconnections .from the :anode and cathode of said diode valve to the cathode ,and lgrid yo Asaid control valve for controlling the conduction of said `control valve in .response to dls- -appearance .of the arc Voltage drop across said rdiode valve, a switching device responsive to the vcurrent of said control valve for reversing the connections of said coil to saidtermmals, and

.an auxiliary switching device controlled by the ycurrent of said control `valve for removing said negative bias and Vreestablishing the connection A'from said potentiometer to the grid of said rst .electric -valve and a timing circuit operatively connected with said auxiliary switching device `for causing said auxiliary switching device to reestablish the connection of `the grid of said rst valveto said'source of negative bias voltage.

of direct voltage, a capacitor and -a potentiom- `eter lconnected in series Vacross 'said source, yswitching means operable .to-one position to ren- -der said capacitor inactive and to -establish a connection from said grid to 'said potentiometer .to initiate conductionin said first valve to maghetize said coil and operable to a second 4position fto render said capacitor active and to connect said vgrid to :a source of negative bias voltage to interrupt conduction of said rst valve land to initiate rdecay of the current in vsaid chuck coil through said diode valve, a lswitching device having :an operating coil and contacts controlled there-by :for controlling the direction of current flow .from said terminals through said magnetizi'ng coil, -a control `electric valve having an anode, cathode and `a control grid and having said operating -coil :connected in its anode-cathode -:circuit and having connections from its -grid and cathode 'to said terminals to cause said control valve to respond to the decay Yof current in said .magnetizing coil substantially to zero to actuate said :reversing switch toreverseithe con-- nections to said magnetizingcoil, yan auxiliary relay controlled by Said control electric valve :for removing said negative bias and reestablishing the connection from ysaid grid of said fust lvalve to said potentiometer to restore vconduction v.of said irst valve, and time delay means for Aeiecting operation of said .auxiliary relay to reconnect the grid of said rst valve to said source of negative bias. i

5. A chuck magnetizing and demagnetizing control system comprising a pair of alternating Voltage supply conductors, first and second chuck magnetizing coil terminals, a rst electric valve provided with an anode, a cathode and a control grid and having its anode connected to one of said supply conductors and its cathvode connected to said first terminal, a connection from said second terminal to VSaid other supply conductor, a diode electric valve having its cathode connected to said rst terminal and its anode connected to said other supplyconductor, a source of direct voltage, a capacitor and a potentiometer connected lin series 'across said source, switching means operable to one position to render said capacitor. inactive and to establish a connection from said grid to--said potentiometer :to initiate conduction in said rst valve to magnetize said coil and operable to a second position to render said capacitor active vand to connect said grid to a source of negative bias voltage to interrupt conduction of said rst valve and to initiate decay of the cur-rent in said chuck coil through said diode valve, a switching device having an operating coil and Areversing contacts controlled `thereby for controlling the direction of current flow from vsaid terminals through said magnetiZi-ng coil, a control electric valve controlled in response to vthe decay of current in said magnetizing coil substantially to zero 'for controlling the energization of said operating coil to reverse said contacts to reverse the connections to said magnetizing coil, and an auxiliary relay controlled by said control electric valve for removing said negative bias and reestablishing the connection from said grid to said potentiometer.

HERBERT F. STORM.

.No references cited. 

